In a paper entitled “Highly Functional Hybrid Modules Using Low Loss Direct Attachment Technique with Planar Lightwave Circuit and LiNbO3 Devices” by T. Yamada et al., “ISMOT-2005”, pp. 107 to 110, a PLC optical circuit and an LN waveguide device are connected together. And further, Japanese Patent Laid-Open Publication No. 2005-173162A describes connecting a PLC optical circuit and an LN waveguide device together. This patent document describes that since the optical waveguides of the two substrates are different in mode field diameter, a tapered portion for adjusting the difference in the mode field diameter is provided between the substrates.
FIG. 3 of Japanese Patent Laid-Open Publication No. H10-133159A shows a Mach-Zehnder optical modulator in which velocity matching between propagating light and modulated signals is made by forming a recess in the undersurface of optical waveguide substrate. That is, the thickness of the optical waveguide substrate is reduced at a harmonic modulating portion.
Moreover, in Japanese Patent No. 3762320B as well, the description is made that in a device with an optical waveguide substrate adhered onto a supporting substrate, a high-frequency interaction portion is reduced in thickness and an incident portion or an outgoing portion is increased in thickness. In this case as well, a recess is provided on the undersurface side of the high-frequency interaction portion, and the recess is filled with an adhesive.